This invention relates to a hydraulic auto-tensioner used to adjust the tension of an engine accessory driving belt.
FIG. 13 shows a belt transmission device for transmitting the rotation of a vehicle engine crankshaft to various engine accessories such as an alternator, a water pump and a compressor of an air-conditioner. This belt transmission device includes a pulley arm 83 provided on the slack side of a belt 81 so as to be pivotable about a pivot shaft 82, a tension pulley 84 supported on a pivot end of the pulley arm 83, and a hydraulic auto-tensioner A that applies an adjusting force to the pulley arm 83 in a direction in which the tension pulley 84 is pressed against the belt 81, thereby keeping constant the tension of the belt 81.
A conventional hydraulic auto-tensioner used as the auto-tensioner A typically includes a cylinder containing hydraulic oil therein, a rod having the lower end portion thereof inserted in the cylinder, a return spring elastically biasing the rod in a direction to protrude from the cylinder, and a hydraulic damper that dampens a pushing force tending to push in the rod.
This hydraulic auto-tensioner further includes a first coupling piece provided at the closed bottom end of the cylinder, and a second coupling piece mounted on a top end portion of the rod protruding from the top end of the cylinder. The first coupling piece is pivotally coupled to engine block, and the second coupling piece is coupled to the pulley arm 83 shown in FIG. 13.
With the auto-tensioner mounted in position as shown in FIG. 13, the first and second coupling pieces tends to displace in the axial direction of coupling shafts extending perpendicular to the axis of the hydraulic auto-tensioner, thus causing the hydraulic auto-tensioner A to be inclined.
If the auto-tensioner A is inclined, the cylinder and the rod cannot be moved smoothly relative to each other in the direction in which the rod protrudes from and retracts into the cylinder. This prevents the hydraulic auto-tensioner to fully perform its expected function. In order to avoid this problem, JP Patent Publication 9-170447A discloses a hydraulic auto-tensioner in which a bush and an elastic ring is mounted in a mounted hole formed in each of a first coupling piece mounted to the cylinder and a second coupling piece mounted to the rod with the elastic ring located around the bush. One of the radially outer surface of the elastic ring and the radially inner surface of the mounting hole is a curved surface such that the elastic ring and the bush supported by the elastic ring are kept in alignment irrespective of the inclination of the hydraulic auto-tensioner.
In the arrangement of JP Patent Publication 9-170447A, in order to prevent separation of the elastic ring and to keep the elastic ring and the bush kept in alignment, the elastic ring has a pair of radially outwardly extending flanges formed at the respective end portions of the radially outer surface of the elastic ring with axial plays left between the flanges and the respective end surfaces of the coupling piece. Since the flanges have an outer diameter larger than the inner diameter of the mounting hole of the coupling piece, the flanges become a major obstacle in mounting the elastic member in position.
The elastic ring has an annular rib on the radially inner surface thereof which is adapted to be engaged in an annular groove formed in the radially outer surface of the bush to inseparably couple the bush and the elastic ring together. But it is not easy to fit the annular rib in the annular groove when inserting the bush into the elastic member.